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Acentric: No centromere.

Acrocentric: See chromosome.

Activating mutation: See mutation.

Activator: A protein that functions to improve gene transcription.

Affordable Care Act: Complex Federal legislation aimed at ensuring that all Americans have practical access to effective health care coverage.

Agenesis: Referring to embryonic formation. Agenesis is the failure of a specific structure to develop. (e.g., agenesis of the corpus callosum means the corpus callosum did not develop at all.)

Hypogenesis: The underdevelopment of a specific structure. (e.g., hypogenesis of the corpus callosum refers to a smaller than normal corpus callosum.)

Allele(s): Alternative forms of a gene, or of a DNA sequence, at a given locus.

Null allele: When applied to a gene that codes for an enzyme, the null allele changes the gene to produce zero enzyme product.

Allelic heterogeneity: See heterogeneity.

Allosteric control: The regulation of an enzyme or other metabolic function via an effector molecule at a site other than the protein's active site (e.g., an allosteric site).

Allosteric protein: A protein that alters its shape upon binding with another molecule.

Alpha-satellite: Repeating units of DNA which are about 170 bp. They are found mainly in the heterochromatic region around the centromeres.

Alternative splicing: The process of excising introns in different patterns to produce alternate DNA forms for transcription and ultimately resulting in different proteins from the same core sequence.

Anaphase: See cell cycle.

Aneuploid: Not possessing the correct number of chromosomes. There are several descriptors of such abnormalities (also see "Ploidy"). Examples would include:

Disomy: Having two copies of one chromosome.

Monosomy: Having one copy of one chromosome.

Trisomy: Having three copies of one chromosome.

Anneal: The coupling of complementary strands of DNA.

Re-anneal: The coupling of complementary DNA strands after having first been separated.

Anticipation: The apparent worsening of a heritable condition as it is passed down through generations.

Apoenzyme: See enzyme.

Apoptosis: Programmed cell death (sometimes referred to as "cellular suicide").

Array comparative genomic hybridization (aCGH): See comparative genomic hybridization.

Association: Two or more malformations, which have been found (on a population basis) to occur together more often than would be predicted by chance, but not necessarily due to a specific causal entity.

Autosome (autosomal): Referring to one of the 22 numbered (non-sex) chromosomes.

Auxology (auxologic): The study of human growth.

Auxotrophy: The inability to synthesize all of the biochemical necessary for all of the functions in an organism. (An auxotroph is an organism that exhibits auxotrophy.)

Prototrophs:...

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